Year-round samples of blood were obtained from wild white-tailed deer. These were assayed for serum testosterone and the results were correlated with antler growth and testicular size. A circannual testosterone rhythm was found with high values in the fall months of August through November (mean 210 ng per 100 ml ± 51 SD), and low values from December through July (mean 34 ng per 100 ml ± 25 SD). Increasing serum testosterone values were correlated with increasing testicular size and hardening and velvet shedding of the antlers. Correlation of this work with previous studies on antler development suggests that the circannual testosterone rhythm is independent of nutrition, social factors, and exposure to females. It may be synchronized to photoperiodic cycles; however, it is present in the absence of photoperiodic stimulation. Specific factors controlling the circannual testosterone rhythm are unknown but appear in some way to be related to circadian rhythms.(Endocrinology94: 1034, 1974)